A Challenge to Debate. III. The Inward and Outward Currents of a Space-Clamped Squid Axon

Abstract

This is the third essay in a series. The first (I) pointed out data that are thermodynamically impossible for the Hodgkin/Huxley (H/H) theory. It also noted that H/H ion-specific gated channels are not susceptible to experimental challenge. The second essay (2) considered why Ohm’s law is not appropriate for action currents and why the Butler-Volmer equation of electrodic catalysis is required instead. Now, and in essay 4, I shall interpret how the Butler-Volmer equation, considered in conjunction with the modulation of a coenzyme, can resolve the thermodynamic challenge in essay 1 and also rationalize the inward and outward currents of a voltage-clamped squid axon. The H/H interpretation of these currents (3) is that there are two kinds of gated channels, sodium-selective channels whose gates open briefly when the excitable membrane is depolarized (thus allowing inward sodium current) and potassium-selective channels, whose gates open subsequently, to allow outward potassium current. In my judgment, there is no evidence that either of these channels exist (4). Even if they do exist, moreover, they provide no rationale for the ability of inorganic and organic ions to substitute for sodium and potassium (4.5). In contrast, electrodics provides a simple explanation of the experimental facts (patch clamp observations included), an explanation that employs no ion-selective channels, gated or otherwise, and that predicates testable properties of bioelectric excitation. Electrodics suggests two electrochemical reactions, reaction BC with its cathode on the peripheral side of the excitable membrane, anode central, and reaction DE, with anode peripheral and cathode central. Thus BC proceeds as an inward current, carried by the predominant mobile external cation (usually sodium), and DE proceeds as an outward current, carried by the predominant internal cation (Fig. I). The sodium and potassium transmembrane concentration differences power neither of these reactions. Instead, both employ electrochemical energy of an organic reaction. Thus we conflict with the widely held opinion among electrophysiologists that organic enzymatic catalysis is too slow for action currents (see ref. 6, for example). The fact